(1) Field of the Invention
The present invention generally relates to a private branch exchange system capable of automatically selecting an optimal communication route coupled thereto.
(2) Description of the Related Art
Recently, a private branch exchange has been practically used which accommodates a plurality of independent tenants and which controls the switching of lines as if the tenants have respective exchanges. Meanwhile, a plurality of communication companies which provide respective communication lines (routes) and equipment have been established. Normally, charges for communications using different communication companies are different from each other. A recent private branch exchange can selectively use a plurality of communication routes. Thus, it is necessary to determine which route should be used each time a call from each tenant is received.
FIG. 1A is a block diagram of a related private branch exchange. A private branch exchange (PBX) has a switch SW to which a plurality of tenants A, B and C are connected. Each of the tenants A, B and C has a plurality of terminals, such as telephone sets. Each terminal is assigned an extension number. The terminals can communicate with each other through the switch under the control of a controller CONT and a memory MEM. Plural trunks, which are coupled to the switch, are provided for each of the tenants A, B and C. The trunks are coupled to communication lines (i.e., routes) such as public telephone lines, in turn coupled to an exchange installed in an exchange office. Normally, the communication lines can be provided by a plurality of communication companies. It is required that the private branch exchange has a function of selecting one of the communication lines in a predetermined way. For example, a least-cost routing method is known.
In order to implement the least-cost routing method, it is necessary to identify which tenant generates a call and which route (communication line) is the least cost route to communicate with a called subscriber in a zone X via, for example, a toll switch. After the least-cost route is identified, a dial number for sending out the received call to the identified route is generated.
In the configuration shown in FIG. 1A, the trunks related to the tenant A are connected to transmission lines (route #1-route #m) provided by a plurality of communication companies. Similarly, the trunks related to the tenant B and the trunks related to the tenant C are respectively connected to transmission lines provided by a plurality of communication companies. The controller CONT selects the least-cost route in response to a call from, for example, the tenant A, and selects a trunk related to the least-cost route. When the selected trunk is busy, the controller CONT selects the second least-cost route and selects a trunk related thereto. During the above-mentioned operation, the controller CONT can recognize the zone or area in which the called subscriber is located from the numerals dialed, and compares the recognized zone with information stored in the memory MEM. By this comparison procedure, the controller CONT can select the least-cost route (optimal route) connected to the recognized zone. Then, the controller CONT generates a dial number in conformity with a network of the identified least-cost route.
In Japan, a special number which discriminates each of plural communication companies from each other is added to the head (i.e., beginning digit positions) of the telephone number. The controller CONT sends to the identified least-cost communication line such a special number and then the dial or dialed number specifying the called party. This will now be described in more detail with reference to FIG. 2.
It is now assumed that a call generated in a zone having a toll number "03" terminates at a subscriber in a zone having a toll number "06" and that four routes provided by four different communication companies are available to connect the toll number "03" zone to the toll number "06" zone. Route #1 directly connects the "03" zone to the "06" zone, while routes #2 -#4 connect the "03" zone to the "06" zone via route #1. Upon receiving the call from the zone "03", the controller CONT recognizes, from the received call, which zone is addressed. Then, the controller CONT determines the optimal route (least-cost route) by referring to a table stored in the memory MEM. If the optimal route is route #1, the controller CONT sends a network of the route #1 the telephone number received from the calling tenant without adding any special number. On the other hand, if the route #2 is determined as the optimal route, the controller CONT adds a company identification number `00aa` to the head of the telephone number received from the calling tenant, and then sends out to the route #1 the telephone number with the company identification number added thereto. The company identification number such as `00aa` is added to the dialed number when a particular trunk is captured and the call outgoes (i.e., is transmitted) via the captured trunk. The route #2 is coupled to the route #1 at an exchange office having, for example, a toll exchange. The toll exchange recognizes the received company identification number and sends out to a network of the route .pi.2 the received telephone number with the company identification number added to the head thereof. Similarly, company identification numbers `00bb` and `00cc` are provided for the routes #3 and #4, respectively.
The configuration shown in FIG. 1A has a disadvantage in that it is necessary to provide, for each of the tenants A, B and C, trunks coupled to different routes or networks provided by a plurality of communication companies.
FIG. 1B shows another related PBX configuration intended to overcome the above-mentioned disadvantage. In FIG. 1B, plural trunks are provided in common to the tenants A, B and C. The trunks are connected to networks provided by different communication companies. When the private branch exchange receives a call from one of the tenants A, B and C, the controller CONT determines the optimal route via the trunks. For example, the controller CONT selects the least-cost route coupled to the destination zone by referring to a table formed in the memory MEM and provided for each zone. If the selected least-cost route is busy, the controller CONT tries to select the second least-cost route.
The table formed in the memory MEM shown in FIG. 1A or FIG. 1B is used in common to the tenants A, B and C. For example, the same least-cost route is always selected in common to the tenants A, B and C. However, in actuality, the needs of the tenants A, B and C may be different from each other. For example, the tenant A needs to use only less-expensive routes, and does not want to be connected to the called subscriber if the less-expensive routes are busy. The tenant B needs to use only expensive routes which have good communication qualities, and does not want to be connected to the called subscriber via less-expensive routes. The configuration shown in FIG. 1A or FIG. 1B does not satisfy such requirements dependent on the tenants.